Polycondensation in solid phase (solid phase condensation) of precondensates produced by melt polycondensation is known. High molecular weight polyesters (based on polyethylene terephthalate) are prepared by subjecting a precondensate (produced in the melt) of average molecular weight to polycondensation at a temperature below its melting point -- that means in solid phase. Suitable processes are disclosed, e.g., in U.S. Pat. No. 3,405,098, in British Patent Specification Nos. 1,004,462 and 1,066,162 and in DOS No. 1,570,844. (DOS = German Offenlegungsschrift.)
Such processes have a series of grave disadvantages. According to U.S. Pat. No. 3,405,098, the precondensate must be ground before it is subjected to solid phase polycondensation. The grain size of the precondensate in this process must be reduced to 0.074 to 0.833 millimeters (mm) and, in addition, at most 10% of the resulting powder may have a particle size of less than 0.074 mm. This requires an elaborate and costly grinding and sifting process. Moreover, by this procedure a pulverized high molecular weight material (which is suitable only to a very limited extent for further processing, e.g. for processing in screw extruders) is obtained. In addition, pulverized material has a very large specific surface and readily absorbs moisture. In extruding polyesters, for instance, such moisture causes a distinct decrease in molecular weight by hydrolytic decomposition. Further, the average molecular weight and intrinsic viscosity increase continuously with time during solid phase polycondensation, but such increases are sufficiently irregular to preclude any accurate prediction of molecular weight after a fixed period of polycondensation time. In individual cases, solid phase polycondensation stops completely after a certain degree of "saturation" is reached.
British Pat. No. 1,004,462 requires starting with ground precondensate having an intrinsic viscosity limited to a stated range, namely: 0.33 to 0.43 deciliters/gram (dl/g), and additionally having a proportion of amorphous phase determined by means of differential thermal analysis. Although intrinsic viscosities of up to 1.5 dl/g are indicated in this patent, a maximum intrinsic viscosity of 0.73 dl/g is actually achieved in the Examples. The same disadvantages encountered in the process of U.S. Pat. No. 3,405,098 also prevail here. The precondensate has to be subjected to additional controlled cooling prior to grinding in order to attain the required proportion of amorphous phase. Moreover, stopping melt condensation at [.eta.]=0.33 to 0.43 dl/g is uneconomical, but is required (when carrying out the process according to this patent) in order to facilitate essential grinding.
British Pat. No. 1,066,162 starts with granules of precondensate and obtains high final viscosities, but only after very long solid phase polycondensation periods. In Examples 2, 3, 4 and 11, reaction times of 193, 66, 153.5 and 191 hours (h) are indicated for reaching intrinsic viscosities of 1.54, 1.36, 1.63 and 1.47 dl/g (determined in 1% solutions of 1:1 phenol:tetrachloroethane at 30.degree. C). [These values are converted from the relative viscosities of the patent specification.] FIG. 1 is a diagram for converting relative viscosity (.eta. rel) to intrinsic viscosity (.eta.).
Although the intrinsic viscosity values thus obtained are high, the reaction times are unreasonably long. This is, perhaps, the major factor in the lack of predictability as to when a certain intrinsic viscosity may be reached. In certain individual cases, solid phase condensation stops entirely after a certain time (see Examples 2, 5, 8, 10 and 11 of the noted British Patent).
DOS No. 1,570,844 provides a process for shortening (by using precondensate particles of definite form and size) the diffusion path of volatile reaction products escaping during solid phase condensation. The particle sizes are from less than 0.2 mm to 2.2 mm. In addition, polycondensate viscosity and average molecular weight is controlled by granular size fractioning and recombining selected granular size fractions of the precondensate. The process, however, requires initial precondensate grinding, preparing particle fractions by selecting certain grinding sizes and, finally, recombining some of the particle fractions for a solid phase condensation batch for achieving a certain desired final viscosity and molecular distribution. A whole series of additional costly working procedures is then required. Moreover, the already nearly pulverized condensate particles are more difficult to process. If, in a further variant of this process, the grinding of the precondensate is omitted and the process starts out with precondensate particles (which are obtained in forming procedures other than grinding) of larger size and definite form, the processing behavior of the larger polycondensate particles is more favorable, but this advantage is gained at the expense of a considerably prolonged polycondensation period.
All of the solid phase condensation processes suggested thus far have the disadvantage that, after having reached a certain condensate viscosity, the viscosity increase per unit time continually decreases.